Soil Testing for Phosphate Rock Application

Soil testing for phosphorus (P) has been a subject for extensive research. Numerous extractants ranging from strong acids to alkalis and various organic and inorganic complexing agents have been developed to evaluate P bioavailability with certain crops and soils. The most widely used soil P tests are Bray I (Bray and Kurtz, 1945), Mehlich I (Nelson et al., 1953), and Olsen (Olsen et al., 1954). Other common tests include Bray II, Mehlich II and III, and resin (Fixen and Grove, 1990). However, all these soil tests are mainly for recommendations with water-soluble P fertilizers such as di-ammonium phosphate (DAP), single superphosphate (SSP) and triple superphosphate (TSP). Reports have shown that these conventional acid or alkaline soil tests do not work well in soils fertilized with phosphate rock (PR) (Perrott et al., 1993; Menon and Chien, 1995; Rajan et al., 1996). Thus, there is a need to develop appropriate soil tests that reflect closely P uptake from PR over a wide range of PR sources, soil properties, and crop varieties. Furthermore, the soil tests should be suitable for PR and for water-soluble P fertilizers. This issue has become more important because of the increasing interest in the use of PR for direct application in developed and developing countries, e.g. Australia, New Zealand, Brazil, Indonesia, Malaysia and countries in sub-Saharan Africa. This chapter provides an overview of the soil tests that have been used to measure available P from soils treated with PR and water-soluble P fertilizer. It provides an introduction to the nature of the reactions of PR and water-soluble P fertilizer in soils. It then discusses the use of the 32P isotopic exchange kinetic method in relation to soil-available P measurement.
Chien, S.H. 2004. “Chapter 6. Soil Testing for Phosphate Rock Application,” IN Use of Phosphate Rocks for Sustainable Agriculture, F. Zapata and R.N. Roy (Eds.), pp 59-68, FAO Fertilizer and Plant Nutrition Bulletin 13, Rome.